Reports water temperature monitoring carried out
as part of an investigation into the possibilities
of the introduced pacific oyster Crassostrea
gigas breeding in British waters. Temperature
recordings (data loggers) were obtained for one
site (Oyster Hut, just W. of west end of Narrows)
approximately 15 times per day from March 1993 to
February >94,
and from two other sites for shorter periods; Ferrybridge
from November >93
to February >94
and Morkham=s
Lake from mid July to mid August 1993.

From March to August, Fleet water temperatures
(at the Oyster Hut) were higher (by approx. 5EC)
than those of mid channel (Channel Light Vessel),
similar in September, then lower (dropping rapidly
by 5-10EC) than mid
channel from October to February. Fleet water temperatures
were within the maximum-minimum range of Weymouth
air temperatures throughout the year. Peak Fleet
water temperature of 17-18EC
occurred in June-August, and minimum water temperature
of 6EC in February.

Other workers (Whittaker, 1978 and data from Abbotsbury
Oyster Farm) indicate maximum water temperatures
in other years of 26-28EC,
and minima in January/February of below freezing,
when the water surface has frozen. No evidence for
vertical stratification has been found, and it is
unlikely in such shallow open water, nor in the
deeper Narrows-Ferrybridge area where tidal currents
are strong.

Depending on the stage in the tidal cycle, highest
and lowest temperatures were recorded during both
night and day. In summer, when Fleet water temperatures
are warmer than sea temperatures, warmer (during
the day) or colder (during the night) water from
the western Fleet flowed past the temperature sensor
at the Oyster Hut. In winter, when sea temperatures
are generally warmer than Fleet temperatures, the
reverse is true. Temperature swings are greater
further west in the Fleet (away from the buffering
effects of seawater exchanged by the tide).

AEven
on neap tides, there was usually some evidence of
a second diurnal cycle at Morkham=s
Lake... this suggests that the statement by Robinson
et al (1988, 668 [should be 1983, 668]) that
>the
semidiurnal tide propagates into the west Fleet
only weakly ... at spring tides, and not at all
... at neaps=
may need some clarification. Robinson et al
(1983, 659) recognise a >fortnightly
fluctuation of water level, related to the spring-neap
cycle, which is strongest towards the western end=,
and state that at neap tide the water >remains
in West Fleet ... and for several days is virtually
isolated from the tide=.
The short record available from Morkham=s
Lake does not indicate an obvious fortnightly temperature
cycle and ... even at neaps a semidiurnal variation
is apparent@.

The paper concludes that in 1993-94, temperature
was not a limiting factor to spawning and settlement
of Crassostrea gigas, but despite this, successful
spawning of this species had not been observed to
date.